1. Field of the Invention
The present invention relates to a safety device, which includes a simple safety element, for protecting a secondary battery from overvoltage and overheating caused by overcharge.
2. Description of the Related Art
Secondary batteries comprehend batteries such as a conventional Ni/Cd (nickel cadmium) battery, a Ni/MH (nickel metal hydride) ion lithium battery and a present lithium ion battery those are capable of charging and discharging. In recent years, the increasing interest in the lithium ion battery has made research and development much more prosperous. That is because the lithium ion battery has much higher energy density than the conventional Ni/Cd battery, Ni/MH battery and so forth. That is, since the lithium ion battery might be fabricated with small size and light weight, it is not only useful for the power source of the mobile device including a mobile phone, a camcorder and a notebook computer, but also considered as the power source of the electric vehicle.
Despite of the above advantages, the lithium ion battery has drawback that it is fragile from overcharge. That is, in case where the secondary battery is neglected without safety device against overcharge, there exists the risk of accidents, resulting in losing the life or the property due to burning out from explosion.
Considering the reaction among the participants forming the battery at the time of overcharge, there is increasing side reaction between LiCoO2, which is active material in the cathode of the lithium ion battery, and electrolyte. Further, the side reaction results in the structural breakdown of the active material in the cathode and the oxidation of the electrolyte. Accordingly, the lithium may be deposited in the active material of the anode consisting of the graphite, etc. If the voltage continues to get higher after the battery reaches the above-said status, it will get exploded or burned out, which result in the accident.
Especially, the situation becomes worse when the voltage standard used in the secondary battery is a high voltage. For instance, in case of connecting the lithium ion secondary battery to the power supply for an automobile, 12V voltage is applied for a car, and 24V voltage for a truck by connecting two 12V power supplies together in series. As such, in case where overcharge is applied instantaneously more than the voltage specified in the standard of the secondary battery, the safety device is desired for protecting the secondary battery against the instant overvoltage.
In the prior art, a charge/discharge control circuit with high price, complex structure, and large occupied area was generally used for protecting the secondary battery against overcharge. In general, the charge/discharge control circuit is constituted such that an input voltage and the battery voltage, or an output voltage to an exterior and the battery voltage are compared to switch the charge/discharge circuit selectively. However, because a control parameter of the charge/discharge control is a voltage, the secondary battery is hardly protected from overheating (temperature). The scheme may be thought to protect the secondary battery from overheating by adding a temperature reacting element such as a PTC or a thermal cutoff. Adding the PTC or the thermal cutoff to the charge/discharge control circuit may contribute to preventing overcharge of the battery and temperature increase of the battery by breaking the applied current on overcharge. However, because the PTC or the thermal cutoff acts above a specified temperature, the temperature of the battery is required to increase above the specified temperature at which the PTC or the thermal cutoff acts. Further, in case where the battery is already on charge, it is difficult to acquire the stability of the battery only with the PTC or the thermal cutoff, which only breaks the applied current when the battery temperature increases due to the external shock, external environment change and internal shortage, etc.